As an AF (auto focus) system of an image pickup apparatus, there is known a TTL phase difference detection system. In the TTL phase difference detection system, part of a light flux from an image pickup optical system is divided into two light fluxes, and a shift amount (phase difference) between two images formed by the two divided light fluxes is obtained. Then, a defocus amount indicating a focus state of the image pickup optical system is calculated based on the phase difference, and a driving amount of a focus lens necessary for approaching the defocus amount to 0 to obtain an in-focus state is calculated.
There has been proposed an image pickup apparatus which performs AF using such a TTL phase difference detection method by providing pixels for the phase difference detection (hereinafter, referred to as focus detection pixels) in an image pickup element to obtain an object image, without providing any exclusive light-receiving sensor for photoelectrically converting the two images.
When the phase difference detection (in other words, focus detection) is performed by using the focus detection pixels while obtaining an image by the image pickup element, the focus detection pixels receive only part of a light flux for normal image pickup, and hence an exposure amount of the focus detection pixels easily becomes low. Especially, when the focus detection is carried out while obtaining a moving image of a low-brightness object, a detection rate (frame rate) of the moving image is high, generally 30 fps to 60 fps, and hence a shortage occurs in exposure amount of the focus detection pixels if the focus detection pixels are exposed at the same rate. Thus, effective sensitivity and a low-brightness limit of the focus detection pixels are lowered.
Japanese Patent Laid-Open No. 2006-261929 discloses an image pickup apparatus which performs, when an image of a low-brightness object is captured, a read-out operation of image signals by alternately switching a live view read-out mode and an addition read-out mode for each frame. In this image pickup apparatus, an image is displayed on a display element by using live view image signals read out in a period of the live view read-out mode, and an AF controller performs focus detection based on added brightness image signals read out in a period of the addition read-out mode.
Japanese Patent Laid-Open No. 2005-295381 discloses an image pickup apparatus which sets image pickup pixels for obtaining a normal image and measuring pixels separately from each other, and reads out outputs from these pixels at different frame rates by different signal processing circuits. In this image pickup apparatus, an exclusive signal line is provided for reading out signals from the measuring pixels.
However, in the image pickup apparatus disclosed in Japanese Patent Laid-Open No. 2006-261929, the live view image signals used for the image displaying and the added brightness image signals used for the focus detection are alternately read out, and hence a frame rate for image displaying is lowered. Further, the addition read-out of the outputs from the focus detection pixels is performed for the low-brightness object, and hence the frame rate for the image display is changed.
Moreover, in the image pickup apparatus disclosed in Japanese Patent Laid-Open No. 2005-295381, the outputs from the image pickup pixels and measuring pixels are processed by the different signal processing circuits, and hence a circuit configuration is complex as compared with a case where signals from the image pickup pixels and measuring pixels are processed by one signal processing circuit.